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• Rhombencephalosynapsis (RES): Congenital continuity (lack of division) of cerebellar hemispheres
○ Complete or partial absence of vermis
○ Varying degrees of midline fusion of cerebellar hemispheres, dentate nuclei
○ May be partial; any portions of hemispheres may be affected

Diagnostic check list
• Small, single hemisphere cerebellum
○ Continuous white matter (WM) tracts crossing midline
○ Absent primary fissure
○ Fused dentate nuclei
○ Small diamond- or keyhole-shaped 4th ventricle
○ ± aqueductal stenosis → hydrocephalus
○ ± corpus callosum dysgenesis (especially posterior)

• Molar tooth malformations
• Lhermitte-Duclos syndrome
• Vermian hypoplasia
• Diffuse cerebellar cortical dysplasia

• Remember to define associated supratentorial anomalies
• Can be mimicked by mechanically induced cerebellar deformation in chronically shunted patients

• Best imaging tool
○ MR

• Protocol advice
○ High-resolution FSE T2WI in multiple imaging planes
– T2WI more clearly defines posterior fossa structures in neonate and infant

• Rhombencephalosynapsis in severe congenital hydrocephalus
• Isolated rhombencephalosynapsis is less common than with supratentorial anomalies

Image Interpretation Pearls
• Can be mimicked by mechanically induced cerebellar deformation in chronically shunted patients

Reporting Tips
• Remember to define associated supratentorial anomalies

Imaging gallery

 Coronal graphic of rhombencephalosynapsis shows that no vermis is present in the midline of the cerebellum. Instead, the folia, interfoliate sulci, and cerebellar white matter are continuous across the cerebellar midline.

 Sagittal T1WI in this child with rhombencephalosynapsis shows absence of the normal vermian lobules and fissures. The cerebellum appears abnormally rounded.

 Axial T2WI in the previous patient shows continuity of the cerebellar folia of both hemispheres across the midline.

  Coronal T2WI shows the absence of a vermis, with continuity of the cerebellar WM and cortex across the midline .

Axial graphic of rhombencephalosynapsis shows the absence of vermis. There is fusion of folia, interfoliate sulci, dentate nuclei , and cerebellar white matter  across the midline.

 Axial NECT in a child with rhombencephalosynapsis shows the cerebellum appears continuous across the midline, without a discernible vermis.

 Axial T2WI in a case of classic rhombencephalosynapsis shows the vermis is absent and the cerebellar hemispheres appear fused across the midline.

More cephalad axial T2WI in the same patient shows the cerebellum appears fused across the midline . Note the abnormal gray matter in the adjacent medial temporal lobes .

 Midline sagittal T2WI in an 11-year-old male with mild mental retardation appears almost normal. 

Coronal T2WI in the previuos case shows the cerebellar white matter ﬇ is fused across the midline.

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